1. Field of the Invention
The invention concerns the provision of electrical contact to Group III-V compound semiconductor devices.
2. Brief Description of the Prior Art
A recurrent problem in the design and fabrication of semiconductor devices is the provision of an electrical contact between the device and external metallic circuitry. The general requirement is that the contact be "ohmic" or nonrectifying so that the properties of the contact itself do not affect the nonlinear properties of the device contacted. The direct application of a metal to a semiconductor, doped to a carrier concentration appropriate to common device use produces a contact which is itself a rectifying diode. In order to solve this problem many techniques have been developed involving alloying or diffusion steps to produce a transition region of heavily doped semiconductor between the active portion of the semiconductor device and the metallic contact. For example, zinc diffusion, to a carrier concentration of approximately 10.sup.20 per cubic centimeter into gallium arsenide, has been used to provide ohmic contact to the p-type portion of gallium arsenide devices (Ripper et al, IEEE Journal of Quantum Electronics, QE6 (1970) 300). This solution to the contact problem requires separate evaporation and diffusion steps subsequent to the processing which results in the formation of the active device.
Recently developed Group III-V semiconductor devices have been produced by the liquid phase epitaxial deposition of succeeding layers of semiconductor in a single processing sequence (Hayashi et al, Applied Physics Letters, 17 (1970) 109). It would be desirable to produce the heavily doped region, required for ohmic contact, by merely providing an extra step in this epitaxial deposition sequence. Although germanium is a known dopant for gallium arsenide, producing p-type conductivity under certain growth conditions (Journal of Physics and Chemistry of Solids, 28 (1967) 2397, Japanese Journal of Applied Physics, 8 (1969) 348), previous work has indicated an inability to produce carrier concentrations sufficiently high (i.e., greater than 3.5 .times. 10.sup.19) to produce ohmic contacts (Journal of Applied Physics, 41 (1970) 264).